We wish to examine the fluorescence of various intracellular and receptor components and their interactions by fluorescence energy transfer. The initial experiments indicated that we could quantitate the fluorescence with the green and blue fluorescent proteins fused to various proteins in intact cells. We have now performed in vivo studies of energy transfer within a cellular membrane. We have also successfully obtained images of expressed green fluorescent protein (GFP) in live monkey kidney cells. GFP fluorescence spectra at selected positions in the membrane have also been measured.